check-commit: spot growing whitespace

We discourage PEP-8-style double blank lines, spot them creeping in.

contrib/check-commit

@@ -1,54 +1,55 b''
 #!/usr/bin/env python
 #
 # Copyright 2014 Matt Mackall <[e-mail]>
 #
 # A tool/hook to run basic sanity checks on commits/patches for
 # submission to Mercurial. Install by adding the following to your
 # .hg/hgrc:
 #
 # [hooks]
 # pretxncommit = contrib/check-commit
 #
 # The hook can be temporarily bypassed with:
 #
 # $ BYPASS= hg commit
 #
 # See also: http://mercurial.selenic.com/wiki/ContributingChanges
 
 import re, sys, os
 
 errors = [
     (r"[(]bc[)]", "(BC) needs to be uppercase"),
     (r"[(]issue \d\d\d", "no space allowed between issue and number"),
     (r"[(]bug", "use (issueDDDD) instead of bug"),
     (r"^# User [^@\n]+$", "username is not an email address"),
     (r"^# .*\n(?!merge with )[^#]\S+[^:] ",
      "summary line doesn't start with 'topic: '"),
     (r"^# .*\n[A-Z][a-z]\S+", "don't capitalize summary lines"),
     (r"^# .*\n.*\.\s+$", "don't add trailing period on summary line"),
     (r"^# .*\n.{78,}", "summary line too long"),
+    (r"^\+\n \n", "adds double empty line"),
     (r"\+\s+def [a-z]+_[a-z]", "adds a function with foo_bar naming"),
 ]
 
 node = os.environ.get("HG_NODE")
 
 if node:
     commit = os.popen("hg export %s" % node).read()
 else:
     commit = sys.stdin.read()
 
 exitcode = 0
 for exp, msg in errors:
     m = re.search(exp, commit, re.MULTILINE)
     if m:
         pos = 0
         for n, l in enumerate(commit.splitlines(True)):
             pos += len(l)
             if pos >= m.end():
                 print "%d: %s" % (n, msg)
                 print " %s" % l[:-1]
                 if "BYPASS" not in os.environ:
                     exitcode = 1
                 break
 
 sys.exit(exitcode)